Phytoplankton blooms and plumes of sediment form the bright blue swirls that ring the Black Sea in this <a href="http://modis.gsfc.nasa.gov" target="outlink">Moderate Resolution Imaging Spectroradiometer</a> (MODIS) image.Sediment flows into the sea through the vast network of rivers that empty into it. The sediment tints the water tan near the mouth of the river and green as the dirt diffuses through the water. In this image, sediment clouds the Sea of Azov in the upper right corner of the image. Two powerful rivers, the Don and the Kuban, flow into the shallow sea, depositing silt in the waters.Sediment is not the only thing that flows into the Black Sea on river water. Some of Europe’ largest rivers also deposit fertilizer from agricultural run-off in the Black Sea, and that feeds frequent phytoplankton blooms. Phytoplankton are microscopic plants that can be seen from space when large colonies grow in one place. Most of the agricultural run-off enters the Sea through the Danube River, the bright green area on the upper left side of the Sea. The water around the river’s mouths is tinted dark green, probably from the growth of surface phytoplankton. The phytoplankton blooms are a problem because their growth and decay can consume all of the available oxygen in a region, creating dead zones where fish cannot live.From the upper left corner, the countries around the Sea are Moldova, Ukraine, Russia, Georgia, Turkey, Bulgaria, and Romania. The <a href="http://modis.gsfc.nasa.gov" target="outlink">Moderate Resolution Imaging Spectroradiometer</a> (MODIS) on NASA’s <a href="http://aqua.nasa.gov/" target="outlink"> Aqua</a> satellite acquired this image on May 22, 2004. Sensor: Aqua/MODIS. Data Start Date: 5/22/04.